A plasma is a special aggregate state, which is produced from a gas. Each gas essentially comprises atoms and/or molecules. In the case of a plasma, this gas is for the most part ionized. This means that, by supplying energy, the atoms or molecules are split into positive and negative charge carriers (i.e., ions and electrons). A plasma is suitable for the processing of workpieces since the electrically charged particles are chemically extremely reactive and can also be influenced by electrical fields. The charged particles can be accelerated by means of an electrical field onto a workpiece, where, on impact, they are able to extract individual atoms from the workpiece. The separated atoms can be removed via a gas flow (etching), or deposited as a coating on other workpieces (thin-film production). Such processing by means of a plasma is used above all when extremely thin layers, in particular in the region of a few atom layers, are to be processed. Typical applications are semiconductor technology (coating, etching etc.), flat screens (similar to semiconductor technology), solar cells (similar to semiconductor technology), architectural glass coating (thermal protection, glare protection, etc.), memory media (CDs, DVDs, hard drives), decorative layers (colored glasses etc.) and tool hardening. These applications make great demands on accuracy and process stability. Furthermore, a plasma can also be used to excite lasers, in particular gas lasers.
To generate a plasma from a gas, energy has to be supplied to the gas. This can be effected in different ways, for example, via light, heat, or electrical energy. A plasma for the processing of workpieces is typically ignited and maintained in a plasma chamber.
To that end, normally a noble gas, e.g. argon, is introduced into the plasma chamber at low pressure. Via electrodes and/or antennas, the gas is exposed to an electrical field. A plasma is generated and is ignited when several conditions are satisfied. First of all, a small number of free charge carriers must be present, in which case, free electrons, which are typically present to a very small extent, are used. The free charge carriers are so forcefully accelerated by the electrical field that as they collide with atoms or molecules of the noble gas they release additional electrons, thereby producing positively charged ions and further negatively charged electrons. The additional free charge carriers are in turn accelerated and, as they collide, generate more ions and electrons. An avalanche effect commences. The discharges produced as these particles collide with the wall of the plasma chamber or other objects and the natural recombination counteract the continuous generation of ions and electrons (i.e., electrons are attracted by ions and recombine to form electrically neutral atoms or molecules). For that reason, an ignited plasma must be constantly supplied with energy in order for it to be maintained.
The supply of energy can be effected via a direct current supply device or an alternating current supply device. The following remarks relate to high frequency (HF) alternating current supply devices with an output frequency >3 MHz.
Plasmas have a very dynamic impedance which makes it difficult to supply uniform HF power. For instance, during the ignition process, the impedance changes very quickly from a high value to a low value. As a result, negative effective resistances can occur during operation, which reduce the current flow as the voltage rises, and undesirable local discharges (arcs) may occur, which may damage the material to be processed, the plasma chamber, or the electrodes.
Power supply devices for plasmas (plasma supply devices) must therefore be designed for a high output power and a high reflected power. EP 1 701 376 A1 has shown that such plasma supply devices can advantageously be achieved by relatively small high frequency amplifiers, the output powers of which are coupled by a coupler, preferably by a 3-dB coupler (e.g., hybrid coupler, Lange coupler, etc.). For that purpose, the two high frequency amplifiers are connected to two ports of the hybrid coupler, hereafter called port 1 and port 2. The high frequency amplifiers are driven in such a way that their high frequency signals of the same fundamental frequency have a phase shift of 90° with respect to one another. At a third port of the hybrid coupler, the first of the two high frequency signals is present lagging by 45°, and the second of the two high frequency signals is present leading by 45°. At a fourth port of the hybrid coupler, the first of the two high frequency signals is present leading by 45° and the second lagging by 45°. By the phase-shifted driving of the high frequency sources, the high frequency source signals thereof add up at the third port by constructive superposition, whereas at the fourth port they cancel each other out (destructive superposition). The high frequency sources upstream of the coupler, thus, each require only half the power of the required high frequency output signal. A cascading of such coupler stages is possible to enable the use of high frequency sources with even less source power or to achieve an even higher power of the high frequency output signal.
The fourth port of the hybrid coupler is normally terminated with a terminating resistance of the system impedance (often 50Ω). As described in EP 1 701 376 A1, a high frequency signal is expected at this port only when a high frequency signal reflected by the plasma load is in turn reflected at the high frequency sources.
In the case of mismatching due to different impedances of plasma supply device and plasma load, the power delivered by the plasma supply device is partially or fully reflected. An impedance matching circuit (matchbox) can transform the impedance of the plasma load in certain ranges and match it to the output impedance of the plasma supply device. If the transformation range of the matching circuit is exceeded, or if regulation of the impedance matching circuit cannot follow a rapid impedance change of the plasma, then the total power delivered by the plasma supply device is not absorbed in the plasma, but rather reflection occurs again.
A particular problem in connection with the described unavoidable reflections of the plasma load is the poor absorption in the system as a whole. Since all components of the plasma supply device and the matching circuit are designed for lowest possible loss in the interests of high efficiency, a high frequency signal reflected by the plasma load travels via an optionally present matching circuit back to port 3 of the hybrid coupler, is here split into two parts and returned via ports 1 and 2 towards the high frequency sources of the plasma supply device. The two parts of the reflected high frequency signal, again, experience in the hybrid coupler an identical phase delay by 45° en route from port 3 to port 1 and an identical phase advance by 45° en route from port 3 to port 2, respectively.
The two high frequency sources can be, for example, two self-contained high frequency generators driven by a common control oscillator. This control oscillator can have a phase shift of 90° for the high frequency signals at its two outputs. The two high frequency sources can alternatively be amplifier stages driven by a common high frequency driver transmitter, the output signal of which is split, for example, by way of a second hybrid coupler. Moreover, the two high frequency sources can also be two ports of a second hybrid coupler having a third port connected to a high frequency generator.
To be able to (i) measure accurately and adjust the high frequency power delivered to the plasma load, even during reflection, (ii) have the opportunity to detect incipient arcs and by suitable measures if possible to prevent them from developing fully, and (iii) supply the optionally present matching circuit with the required information about the impedance of the plasma, is it desirable to know all relevant high frequency operating parameters that arise between high frequency generator and plasma load or matching circuit. These include, for example, the power Pf of the high frequency output signal delivered by the plasma supply device, and also the high frequency operating parameters influenced by the complex impedance of the plasma load and optionally the matching circuit, such as power Pr and phase angle φ of the reflected high frequency signal, and the variables dependent thereon, such as reflection factor,
                  Γ              =                            P          r                          P          f                      ,or the total power of the high frequency output signal and reflected high frequency signal,PS=Pf+Pr=Pf·(1+|Γ|2),as well as the power of the high frequency signal reflected by the high frequency sources for a second time. In the knowledge of these high frequency operating parameters, the matching circuit can be driven, the power of the high frequency output signals can be adjusted, and the state of the plasma can be reliably determined.
Directional couplers may be used to measure the powers of the high frequency output signal and the returning high frequency signal. A directional coupler necessitates an expensive component, which, even though accuracy should be high in the case of large proportions of reflected high frequency power, requires especially narrow manufacturing tolerances. In addition, to determine the phase, the two measured signals need to be combined before their detection. This can be effected, for example, by vector analysis with or without preceding down-mixing into a different frequency range or into the baseband or by mixing their standardized oscillations. Both methods are quite complex.
U.S. Pat. No. 4,489,271 discloses an arrangement with which the phase angle of the reflected high frequency signal can be also determined. However, this arrangement requires five different couplers.
Another possible method of measuring the output delivered to the load is to measure the current and the voltage. In that case, however, a very good measurement of the voltage and the current sensors must be achieved. Here too, to determine the phase angle φ and the reflection factor Γ, a phase comparison of the standardized high frequency measurement signals of current and voltage in a high frequency mixer is required, for example, or a vector analysis of these two measurement signals.